In general, when a pump is suddenly stopped or a valve closes suddenly in a water pipeline system, a transient condition in which an amount of flow and a hydraulic pressure are dramatically changed occurs. This phenomenon is referred to as water hammer or fluid hammer.
As a result of water hammer, pressure inside a pipe is dramatically increased or is decreased to be lower than a saturated vapor pressure such that vapor occurs, and after that, in the process of column separation and return, the pipe may be broken or damaged by a shock wave.
For example, as shown in FIG. 1, a water pipeline system includes: a water supply pump 2 supplying water from an inlet 1 to one direction; a main pipe P with the water being transferred therethrough; and an outlet 3 for discharging the water transferred from the main pipe P.
Further, the main pipe P may be provided not only with a check valve 4 for preventing back flow, but also with a flexible joint for preventing vibration and a shut-off valve for controlling the water flowing through the outlet 3.
Here, when the water supply pump 2 is stopped or the shut-off valve is quickly closed, velocity of flow is dramatically changed in the main pipe P between the inlet and the outlet 3, and thereby water hammer occurs, damaging the main pipe P or the water supply pump 2.
For the above reason, as shown in FIGS. 2 and 3, according to a document of related art, such as Korean Patent application publication No. 2013-0093299, a check valve is configured such that a shock-absorbing damper 50 is connected to a rotation shaft 20 of the disc 30, whereby in the case of quick closing of the valve, noise and vibration caused by a collision between a disc 30 and a valve body (for example, a valve seat surface), and water hammer caused by dramatic change in velocity of flow are prevented.
Thereby, as shown in FIG. 2a and FIG. 2b, even when the pump is stopped and the disc 30 moves down by weight of a balance weight 40, the disc 30 is slowly closed, that is, a slow closing function is provided by the shock-absorbing damper 50 constituted by a hydraulic cylinder, and the like.
However, as described above, when the slow closing function is applied, since the disk closing speed is slowed from the beginning of the disk closing, it not only allows a large amount of backflow but also increases the backflow time and also increases the backflow speed. Further, as the disk is accelerating as it closes, at the last moment the disk is closed, the backflow speed increases significantly, and more serious consequences than a quick-closing check valve may be caused due to the acceleration of the disk.
In other words, if it takes long for the disc 30 to move down to close a passage, during the time, a massive amount of fluid (namely, water) flows backward, and accordingly it is impossible for the check valve to perform its function. Moreover, the back flow may apply a pressure to the stopped water supply pump 2, or may cause a problem by making the water supply pump 2 rotate reversely.
Accordingly, when quick closing of a check valve is performed, noise, vibration, and water hammer occur. On the contrary, when slow closing is performed to solve this problem, the closer the disc is to being closed, the more force is exerted by the reverse flow, and at the end, the closing speed of the disc increases greatly, and the velocity of the backflow increases. When the velocity of the backflow increases, as can be seen from the following equation 1, variation of backflow velocity (ΔV) increases, and variation of head (ΔH) also increases, which leads to an instantaneous increase in pressure inside a conduit, whereby water hammer occurs.ΔH=(C/g)ΔV  (equation 1)
(Herein, H: head, C: shock wave transmission rate according to pipe material characteristics, g: gravitational acceleration, V: backflow velocity)
Since water hammer cannot be sufficiently mitigated by the installation of the air chamber, the improvement of the check valve is required.
In particular, when the main pipe P is provided with an air chamber 3 to prevent water hammer, the air chamber 3 is expensive, and may accelerate the quick closing of the check valve by a hydraulic pressure caused when make-up water stored in the air chamber 3 is supplied to the main pipe P when the pump is suddenly stopped.